The purpose of this proposal is to investigate mechanisms of Gamma hydroxy butyrate (GHB) tolerance and dependence after chronic administration of low and high doses of GHB to rats. GHB is a novel sedative-hypnotic that is an emerging drug of abuse. GHB activates GHB, GABA-B and possibly GABA-A receptors, with a unique dose response relationship for each. When recreational users escalate use, tolerance develops and a withdrawal syndrome can occur that is characterized by insomnia, anxiety, and hallucinations. Tolerance and dependence to GHB are poorly characterized in animal models. We hypothesize that tolerance is related to dose and duration of exposure. We also hypothesize that the different receptor populations adapt at varying rates. We postulate that chronic treatment with lower doses or shorter regimens will cause tolerance at GHB and perhaps GABA-B receptors, while higher doses and longer treatments will lead to marked tolerance to GHB, GABA-A and GABA-B receptors. We will assess tolerance to GHB effects on sleep time, tilt plane and plus maze performance after chronic treatment with low or high doses for 7, 14 or 21 days. We will assess spontaneous and GHB (NCS-382) and GABA-B (CGP46381) antagonist-precipitated withdrawal by measuring sleep-wake cycle, locomotion as well as blood pressure and heart rate. Cross-tolerance to GABA-B (baclofen) and GABA-A (pentobarbital, diazepam) agonists will be assessed with the same behavioral measures. We will characterize inhibitory GHB mechanisms using electrophysiologic techniques in frontal cortex. GHB effects on spontaneous, evoked and mini GABA-A IPSCs and on postsynaptic potassium conductance will be determined. The effects of low and high GHB concentrations will be contrasted, and blockade by NCS-382 and CGP46381 on all parameters will be determined. Tolerance to specific GHB and GABA-B mechanisms will be studied by evaluating the same parameters in frontal cortex slices from animals treated chronically with low or high dose GHB. Finally, we will assess GHB effects on GABA-A receptors in naive and tolerant animals by measuring effects on GABA-mediated C1 uptake into synaptoneurosomes and its modulation by benzodiazepines, barbiturates and neurosteroids. These experiments should ultimately lead to the development of more effective pharmacotherapies for GHB dependence, which is an emerging drug abuse problem.